In certain devices, three piece inflatable penile prostheses (IPP's) consisted of an abdominal reservoir, a pair of inflatable penile cylinders and a scrotal pump to transfer fluid between the reservoir and cylinders. The pump may require repeated collapsing of the pump bulb to transfer fluid from the reservoir to the penile cylinders to achieve an erection. To deflate the cylinders and return the penis to the flaccid state, the pump valve mechanism can be squeezed until the desired amount of fluid is transferred.